The present invention pertains to prosthetic devices. More particularly, the invention pertains to knee joint prostheses which may be surgically implanted between the femoral condyle and tibial plateau of the knee joint.
Articular cartilage and meniscal cartilage provide the mobile weight bearing surfaces of the knee joint. Damage to these surfaces is generally due to genetic predisposition, trauma, and/or aging. The result is usually the development of chondromalacia, thinning and softening of the articular cartilage, and degenerative tearing of the meniscal cartilage. Various methods of treatment are available to treat these disease processes. Each option usually has specific indications and is accompanied by a list of benefits and deficiencies that may be compared to other options.
The healthy knee joint has a balanced amount of joint cartilage across the four surfaces of this bi-compartmental joint (medial femoral condyle, medial tibial plateau, lateral femoral condyle and lateral tibial plateau). In patients with osteoarthritis, degenerative process typically leads to an asymmetric wear pattern that leaves one compartment with significantly less articular cartilage covering the distal portions (or weight bearing area) of the tibia and femur than the other compartment. Most commonly, the medial compartment of the knee joint is affected more often than the lateral compartment.
As the disease progresses, large amounts of articular cartilage are worn away. Due to the asymmetric nature of the erosion, the alignment of the mechanical axis of rotation of the femur relative to the tibia becomes tilted down towards the compartment which is suffering the majority of the erosion. The result is a Varus (bow-legged) deformity in the case of a medial compartment disease predominance, or a Valgus (knock-kneed) deformity in the case of lateral compartment disease predominance. Factors such as excessive body weight, previous traumatic injury, knee instability, the absence of the meniscus and genetic predisposition, all affect the rate of the disease.
The disease is usually defined in stages of Grade I through V, with Grade III revealing significant articular cartilage loss, Grade IV revealing some eburnation of the subchondral bone, and Grade V detailing both significant articular loss and bone loss.
It is important to understand that the disease manifests itself as periodic to continuous pain that can be quite uncomfortable for the patient. The cause of this pain is subject to many opinions but it is apparent that, as the joint compartment collapses, the collateral ligament on the side of the predominant disease becomes increasingly slack (like one side of a pair of loose suspenders), and the tibial and femoral axes move, for example, from a Varus to a Valgus condition. This increases the stress on the opposing collateral ligament (and cruciate ligaments as well) and shifts the load bearing function of this bi-compartmental joint increasingly towards the diseased side. This increasing joint laxity is suspected of causing some of the pain one feels. In addition, as the bearing loads are shifted, the body responds to the increased loading on the diseased compartment with an increased production of bony surface area (osteophytes) in an attempt to reduce the ever-increasing areal unit loading. All of this shifting of the knee component geometry causes a misalignment of the mechanical axis of the joint. The misalignment causes an increase in the rate of degenerative change to the diseased joint surfaces causing an ever-increasing amount of cartilage debris to build up in the joint, further causing joint inflammation and subsequent pain.
Currently, there is a void in options used to treat the relatively young patient with moderate to severe chondromalacia involving mainly one compartment of the knee. Current treatments include NSAIDs, cortisone injections, hyaluronic acid (HA) injections and arthroscopic debridement. Some patients cannot tolerate or do not want the risk of potential side effects of NSAIDs. Repeated cortisone injections actually weaken articular cartilage after a long period of time. HA has shown promising results but is only a short term solution for pain. Arthroscopic debridement alone frequently does not provide long lasting relief of symptoms. Unfortunately, the lack of long term success of these treatments leads to more invasive treatment methods. Osteochondral allografts and microfracture techniques are indicated for small cartilage defects that are typically the result of trauma. These procedures are not suitable for addressing large areas of degeneration. In addition, osteochondral allografts can only be used to address defects on the femoral condyle. Tibial degeneration can not be addressed with this technique. High tibial osteotomy (HTO) corrects the varus malalignment between the tibia and femur but, because it is performed below the joint line, it does not fill the cartilage void or re-tension the medial collateral ligament (MCL). Removing bone and changing the joint line does not complicate the conversion to total knee arthroscopy (TKA). However, an HTO does leave a hard sclerotic region of bone which is difficult to penetrate making conversion to a total knee replacement (TKR) technically challenging. Unicompartmental and bicompartmental total knee replacements resect significant amounts of bone and, if performed on younger patients, will likely require revision surgery as they age. Revision total knee replacement surgery is usually extensive and results in predictably diminished mechanical life expectancy. Therefore, it is best to delay this type of bone resecting surgery as long as possible.
The only true solution is to rebuild the defective joint by “filling” the joint space with more articular bearing material through a complete resurfacing of the existing femoral condyle and tibial plateau. By replacing the original cartilage to its pre-diseased depth, the joint mechanical axis alignment is restored to its original condition. Unfortunately, these natural articular materials and surgical technology required to accomplish this replacement task do not yet exist.
Currently, replacement of the existing surfaces, with materials other than articular cartilage, is only possible with a total or uni-condylar knee replacement, and these procedures require removal of significant amounts of the underlying bone structure.
The alternative method is to fill the joint space with a spacer that replaces the missing articular materials. This spacer should also provide an anatomically correct bearing surface for both the tibial and femoral surface (U.S. Pat. No. 6,206,927).
Attaching a new bearing surface to the femoral condyle is technically challenging and was first attempted, with limited success, over 40 years ago with the MGH (Massachusetts General Hospital) knee. Like a dental crown, it covered both femoral condyles with Vitallium (CoCr) and would bear against the existing tibial plateau.
Tibial covering devices such as the McKeever, Macintosh and Townley tibial tray, maintained the existing femoral surface as the bearing surface, but like the MGH knee, all required significant bone resection, thus making them less than ideal solutions as well.
These devices also made no particular attempt to match the patient's specific femoral or tibial geometry thus reducing their chances for optimal success. Because these devices were made of CoCr, which has different visco-elastic and wear properties from the natural articular materials, any surface geometry which did not closely match the bearing surface of the tibia or femur, could cause premature wear of the remaining cartilage due to asymmetric loading.
Newer materials technologies in development include filling the joint space by injecting polyurethane (U.S. Pat. No. 5,795,353) into the joint and anchoring it with holes drilled into the tibial plateau. Others include a series of polymeric materials such as PVA Hydrogels in a titanium mesh as described by Chang et al, “Historical Comparison of Tibial Articular Surfaces Against Rigid Materials And Artificial Articular Cartilage,” Journal of Biomedical Material Research, 37, 51-59, 1997, biodegradable anhydride prepolymers that can be cross linked with irradiation by UV light (U.S. Pat. No. 5,902,599) and in-vivo grown articular chondrocytes in a collagen fiber or other bio-compatible scaffold (U.S. Pat. No. 5,158,574). Other low surface energy materials, such as low temperature isotropic (LTI) pyrolitic carbon, have been investigated as bearing surfaces as well.
All of these techniques are limited by one's ability to first of all fashion these materials in a conformal fashion to replicate the existing knee geometry, while at the same time, maintaining their location within the joint while further being able to survive the mechanical loading conditions of the knee.
Therefore, what is needed is a uni-compartmental interpositional spacer which, by effectively replacing worn articular material, restores normal joint alignment without requiring any bone resection and provides an anatomically correct bearing surface for the femoral condyle to articulate against.